512 



COMPARATIVE ANATOMY 



CHAP. 



In the body of the Arachnoidea we find almost always a number of 

 anterior segments, probably 7, fused to form a generally unsegmented 

 eephalo- thorax. Following upon this cephalo- thorax there is an 

 abdomen consisting of a varying number of separate or fused segments, 

 which may again fuse with the cephalo-thorax, as is the case in the 

 Acarina and Linguatulidce (1) and thus the body appears neither segmented 

 nor divided into regions. We thus, within the class of the Arachnoidea, 

 have not only fusing of segments and an obliteration of segmentation, 

 but also a progressive concentration of the whole body ; there can be 

 little doubt that the Arachnoidea are no exception to the rule that the 

 more richly and completely segmented the body is the better has it 

 retained the primitive arrangement. The segmentation is richest in 



the Scorpionidce and Solpugidce, but 

 is very different in the two groups. 

 The segmentation in both these 

 forms claims special attention. 



The Solimgidce (Fig. 358) vividly 

 recall the Insecta in the metamerism 

 of the body. In opposition to all 

 other Arachnoidea, not only is the 

 anterior division of the body, 

 answering to the cephalo-thorax, 

 distinctly segmented, but it even 

 falls into two parts, an anterior 

 unsegmented head, which may be 

 compared with the head of the 

 Antennata, and a posterior thorax, 

 consisting of three segments, which 

 may be compared with the thorax 

 of the Insecta and with the three 

 anterior trunk segments of the 

 Myriapoda. An abdomen of ten 

 segments follows the thorax. 



There are considerable difficulties in 

 the way of comparing the segments of the 

 body of the Solpucjidce with the head and 

 a corresponding number of trunk segments 

 of the Antennata ; these difficulties arise 

 chiefly in comparing the extremities and 

 nervous system, and will be discussed later 

 on. The ontogeny of the Solpugidce is 

 unfortunately almost unknown. 



In the Scorpionidce (Fig. 359) 

 the cephalo-thorax is, in the adult 

 condition, unsegmented ; in the 

 embryo, however, a segmentation into seven somites, including the 

 frontal lobes, may be recognised. The abdomen, on the contrary, 



Fig. 359. Scorpio africanus (after Cuvier, 

 Regne animal). 



